


Crimson Beast

by RoseDarkfire



Category: Underfell - Fandom, Undertale (Video Game)
Genre: Alternate Universe- Underfell, Double Penetration, F/M, Gaster Hound - Freeform, Pet Names, Underfell Sans (Undertale), beastiality, reader is female, safe words
Language: English
Status: Completed
Published: 2020-04-21
Updated: 2020-04-21
Packaged: 2021-03-02 02:46:45
Rating: Explicit
Warnings: Creator Chose Not To Use Archive Warnings
Chapters: 1
Words: 4,531
Publisher: archiveofourown.org
Story URL: https://archiveofourown.org/works/23777839
Author URL: https://archiveofourown.org/users/RoseDarkfire/pseuds/RoseDarkfire
Summary: Sans found out one of your Kinks. And now he was using that information on you.
Relationships: Sans (Undertale)/Reader
Comments: 12
Kudos: 139





	Crimson Beast

**Author's Note:**

> Hi Folks!
> 
> So I had this weird little idea in my head the other day. I asked my friend if I should write it and she said go for it. 
> 
> *Rubs the back of her neck* I really had fun writing this. 
> 
> Enjoy!

You hadn’t seen Sans for two weeks since you had been on nonstop graveyard shifts. You knew that he would snoop since he was curious about what things you were into. You had willingly explored a lot of his kinks when the two of you had hooked up. Still did when he found something new that he wanted to try. Lu didn’t know that when she let him into the apartment that he would go snooping. You recalled walking into your apartment finding him reading one of your werewolf hentais. Just sitting there casually drinking mustard on your couch flipping the pages. Your roommate had been sitting across from him in the recliner reading a copy with a summoner being done by a fire elemental. 

  
  


You had marched over to the couch snatching it out of his hands, then doing the same to Lu. Her face turned about as red as yours had before you had recovered. When you turned to face your boyfriend that sharp grin of his was so wide while his sockets were half lidded. You could see the plan forming in his skull as bright as his red eye lights. He didn’t mention it after you had put them away. Sans went to acting like normal, making sure to slide his hand over your ass every time you walked by him. 

  
  


Sans had made a sexual joke towards your roommate. You didn’t mind he wouldn’t get far with her. Plus Lu expressed that she only saw Sans as a friend. If he did well you might have to just let him suffer that point. Lu’s appeared as the sweet neighbor that makes sure to take on pretty please when she comes over to ask for sugar. But she would have you whimpering those words if you ever booked a session with her. You had been in the apartment during one of her sessions. She liked to be called Mommy when she spanked her boys. One drunk night you had dared to ask her what it was like having sex in a dominant role. Her response was not something you didn’t expect. 

  
  


“Oh I’ve never had sex before nor is it really my kink to be in charge. That just pays my rent. They don’t want to have sex honestly. They just want me to tell them what bad boys they are while they ride dildos. I am usually behind them minstrating the toys. I am waiting for the right guy to come along.” And she said it in the same voice she asks for sugar in. It was scary.

  
  


That had been a very informative night. You both knew about each other's strange kinks. She knew your fetish for a werewolf to mount you and practically plow you into the ground. She had this fetish for having sex outside and under the stars. She actually liked reading your books and you had seen her hanging around Chillby’s. She liked to hang around with Chillbz. 

  
  


So you hadn’t really expected anything less from her but you had asked her why she let Sans in your room when he left. She stated she wasn’t aware that your bedroom was off limits. And yeah you had never stated that your boyfriend who came over on a regular basis wasn’t allowed in your bedroom. So this was all on you, and you weren’t sure that you were looking forward to what your boyfriend would do with that information. You just knew that he was going to do something with it, you could feel it in your gut.

  
  


He texted you a couple days later asking for you to come over to his place. You told him that you had a long shift. When he asked when the next time you could spend the night would be you told him maybe in a week or so. It wasn’t a lie you did have to fill some shifts at the store since one of the late shift people quit. 

  
  


Tomorrow was your day off and Sans had been silent about this surprise that he has in store. Sans had given you instructions that you could enter his home. Something that he had told you before that unless you had his permission not to do. In the underground, if you entered a monster's home without permission then you would be dusted. It was a rule that you learned the hard way when you ran into Sans’ younger brother Papyrus. He had berated Sans about not teaching you about monster etiquette. From there you learned about it, Sans didn’t want anything happening to you after all. 

  
  


Now you were standing in his living room looking around. Papyrus must have cleaned up already. You knew it had been dirty the last time that you had come over. It was actually nice that the younger brother took care of Sans. Then again they lived together so you guessed they had to look after one another. “Sans?” You called out wondering where he was.

  
  


“bout time ya showed up sweetheart.” Sans spoke in his deep baritone voice that was littered with a Bronx accent.

  
  


You spun and found him resting up against the railing of the stairs. He wasn’t wearing his signature black leather jacket. He was dressed in just his thick red turtleneck sweater and black shorts. You could see the fuzzy black slippers that he was wearing. There were little gold skull charms attached to the tops of them. He was wearing one red sock and one black sock.

  
  


“I told you I was working late shifts.” You explained again watching him fold his arms over his chest. You knew that your boyfriend was insecure. It was one of the reasons you made sure to spend the night every chance you could. Only going home before and on days you worked. “I came over like I usually do before my days off.”

  
  


“how many ya got off?” Sans asked after a bit.

  
  


“Um....” You paused to think about it a moment. “About three. My manager gave me an extra day since I’ve been a good sport about Kenny quitting.”

  
  


“good.” You looked up in time to see Sans finish the short walk to you. His hands sliding out from the pockets of his shorts. Looking you over he must have seen how tired you were. “c’mon let’s get ya some good night rest.”

  
  


Sans led you upstairs giving you a chance to shower while he fetched your sleepwear. You had packed a bag with a few sets of clothes so that you only had to come over after work. Showered, clothed, and teeth brushed you walked into Sans’ bedroom, being mindful of the piles around it. Mixes of socks, mustard bottles, and various notes that he had been unsatisfied with. Joining Sans in the bed he snuggled you to him burying his nasal cavity against your neck.

“‘ve missed ya sweetheart.” He sounded much more at ease than when you had seen him downstairs. Even though you made sure to send him screenshots of your schedule to ease his mind that you weren’t avoiding him. He still tended to get scared that you didn’t want to be around him. Sometimes he was a handful but he was worth it. 

  
  


“yanno that ‘m gonna take full advantage of ya while yur here.” You heard him say between a long yawn. Yeah you were aware that he would do something like that. It’s been two weeks since you last played. What worried you though was that he might want to talk about your wolf kink. 

  
  


“I figured. Just make sure you give me a day to recover if you decide to go too far.” You asked watching his right socket open to look up at you. 

  
  


“ya know safe words will always be an option for ya, doll. wouldn’ do my girl wron’.” He leaned up kissing your cheek. “now get some rest for me. i need ya well rested.” You followed his command instantly falling into an exhaustion induced sleep. The last thing you had seen was your boyfriend's face wearing that same look you had seen at your apartment. 

  
  


You were jolted awake from something prodding against your slit. You attempted to wiggle away from whatever was pressing into you. But you found your legs unable to move forward, they were effectively blocked by the bed. You were flipped over on your stomach with your hands bound underneath you. Your legs were pulled off the bed so that your knees were being pressed against the floor. You could feel whatever was prodding your sex delve deeper. It made you gasp in a sharp breath of air.

  
  


You heard a low growl sent in your direction. “bout time was wonderin’ if i would have to ride ya hard to get ya to wake.” Sans made another swipe at your entrance getting you to wiggle against his mouth. 

  
  


Sans reacted by slapping a hand against your ass causing you to whine. “don’ be a brat.” That had you moaning in response loving when he got a little rough with you. “‘m tryin’ to give ya that surprise ya have had waitin’ fer two weeks. now stop wigglin’ an’ just enjoy. gonna needja nice an’ wet.”

  
  


He made this slurping sound that made you feel self conscious of what he was doing. It was usually hard for Sans to ever make you feel that way. But hearing the way he was going to town on your juices made you shiver. You weren’t much for sharing certain interests with him because you didn’t want him thinking you were weird. But you had no qualms doing his. Something about making him satisfied always warmed your chest. 

  
  


The press of his tongue just on the entrance of your slit always felt amazing. Sans was big on making sure that you were well prepared for whatever he had in store. He even made sure to stock up on Lube for when he went into heat. Stated that his heat was a real test of his patience just wasn’t there when it came to doing the deed. Just feeling the way that the flat of his tongue slid up your slick folds had you whimpering. 

  
  


His  _ ecto  _ as he called it had this ability to feel warm when first pressed against any part of her body. Then the longer it stayed connected it got hotter. It never got painful though, it was kept at just a pleasurable heat. 

  
  


“Sans!” You whispered out into the air when he rolled his tongue inside of you. Moving it in a manner it wiggled against the ways. Slowly fucking you into a whining mess. You couldn’t help but wiggle your hips wanting to feel other parts of him inside of you. “Please Sans!” You whined.

  
  


Thankfully Sans pulled his tongue out of your slit causing you to pant heavily. It gave you a small reprieve but you felt so empty still. You wanted his smoky cherry flavored cock shoved deep inside of you right now. “Sans please fuck me!” You begged him again. 

  
  


“is that my name, pet?” Sans hissed at you making you cringe within yourself. No that was not the name you were supposed to call him.

  
  


“Daddy.” You spoke keeping an innocent lilt to your voice while you called out to him. He gave off a satisfied purr that had your body worked up all over again. You knew that he was loving the fact you had done it because he pressed his pelvis to your backside. Allowing the clothed bulge of his cock press against your entrance. “Please Daddy, I want you…” Another thrust and you could feel the head pressing into your entrance slightly. 

  
  


“what does my cute little pet want?” He hummed pulling his hips back making you nearly sob.  _ No come back! I want you to fuck me! _

  
  


“Daddy I want to be fucked!” You cried loudly getting him to reach over you sliding his phalanges over your mouth.

  
  


“careful pet. paps may not be here right now with bein’ out with undyne. but he is likely to show up. an’ if he hears ya cryin’ such profanities he might wanna find out what ya feel like.” Sans chuckled darkly. You knew that he wouldn’t let his brother do anything to you. Sans was a very possessive skeleton when it came down to it. The idea of sharing you with anyone was a big no no. It’s why you didn’t make a big deal he liked to spend time with Lu’s because he didn’t like to share.

  
  


“sides i got somethin’ real special for my pet tonight.” You heard him snicker moving his hips to press against your front. Giving you one last thrust before he got to his feet. He didn’t remove his phalanges from over your mouth as he walked to the front of you. 

  
  


Your gaze dipped down to the red tinted glow that was coming from the front of his boxers. You couldn’t help but lick your lips in response to seeing it. “does my little pet want daddy’s cock?” Sans asked, getting you to flick your gaze back up to him. You nodded your head mutely in response to his question.

  
  


Sans glowered down at you. “if yur gonna not do as yanno yur ‘sposed ta then ‘m just not gonna give ya an orgasm.” Sans tsked making you want to whimper.

  
  


The bedroom rules were simple. Do as Sans asked. Always use Daddy when he was teasing or fucking you. And ALWAYS express what you wanted in a vocal manner. You were actually trying to get in trouble. Sans’ punishments were always so much fun. And made for your days off. You liked to play the spoiled little brat that would fight him all the way. 

  
  


Sans sighed moving to sit back on the bed but his grin was wide while he reached up. Taking hold of the spiked collar that was around his neck. You watched him undo the buckle and slid it off. Leaning forward he placed it around your neck. Not even really tightening it too much. He only put the collar on you when he wanted full control over the situation. “yur bein’ a real brat today. not callin’ me by my bedroom name when ya want somethin’. won’ sit still. an’ now yur not vocalizin’ what ya want.”

  
  


His voice was calm while he spoke to you but you noticed that his left eye light was brighter than the other. A good signal that he was using his magic. You could feel it vibrating in the air. There was a light tapping sound behind you making you turn your head. Sans yanked on the collar making you look forward. “did i say that ya could look behind ya?” 

  
  


You shook your head but you could pick up on that clicking sound becoming muffled. Whatever it was, it was on the carpet. “i know yur rather reserved with yur kinks, pet. but i hadn’ thought why that might be. there’s nothin’ wron’ with a beastiality kink. wasn’ expectin’ that though.”

  
  


Oh stars, he wanted to talk about this now? “i know ‘m not capable of lookin’ like them furry beasts that ya read about. but with that said i am pretty good at gettin’ creative with my magic.” He grinned down at you flashing that gold tooth on the bottom row of those shark teeth. You could make out what was behind you. 

  
  


You fought with Sans but after a bit he let go of the collar letting you turn your head. A gasp escaping as you came face to skull with one of Sans’ gaster blasters. They were dragon skulls with four glowing sockets that were locked on you. Its maw opened to allow a long red tongue stretch out and glide up your cheek. There was a slick feeling to it but it started to warm up much like when Sans had gone down on you. 

  
  


Sans grabbed your shoulders pulling you so that your legs were firmly pressed against the mattress. “best sit still pet. not sure how aggressive he’s gonna get. but don’ worry i do have control over him.”

  
  


Control over what exactly? You saw the blaster move to reveal that it was standing on all fours. It was all bones from what you could see. It’s spine had sharp spinous processes that extended down to it’s tail that was swaying back and forth behind it. You watched it move its front legs up it’s claw appearing more poseable than they should be. He made a Gaster Hound that was similar to the werewolves in your hentai. Wait, Sans called it a him. Your attention became focused on the fact that it’s ribcage was giving off a dim crimson colored glow. It moved through the bones that made up in its body all the way down between its hind legs. Oh it had a… you connected with what Sans had made this thing for.

  
  


Turning your head you looked up at him seeing that his zygomatic bones were flushed with color. He held his hand up to keep the blaster hound from continuing. “i heh… wanted to do somethin’ nice for ya honestly. ya’ve been such a good girl an’ actually participatin’ in daddy’s kinks. so why don’ we test one of yur own.” He reached down to stroke your cheek. “color, pet?”

  
  


You were rendered speechless. He had waited two weeks for him to do something like this for you. And he actually used his magic to bring your kink to life. “How does this work exactly?”

  
  


“it’s mostly my magic. i admit watchin’ ya be fucked by one of my blasters is gonna be hot. gonna be gettin’ my rubbin’ on while it happens. it’s probably gonna drain me for the evenin’ though. but it’s gonna be so worth it.” Sans leaned down pressing his teeth against your lips making you moan. It was all his magic which meant he would be able to feel this.

  
  


Moving out of the kiss you tilted your head to look up at him. If he made this for you the least that you could do was make it easier on him. “Let me take care of you too Daddy. You don’t have to rub it out, let me give you head?” You asked him, getting Sans’ sockets to widen in surprise to what you had just asked.

  
  


“heh ya don’ have to do that pet. i need to make sure sparky here is lubed a bit more. don’ wanna have him hurtin’ ya. he’s bit bigger an’ he’s very much like them werewolves ya read about.” In other words Sans gave Sparky an actual bulbus glandis and that made you shiver excitedly. Sans slid off the bed going into one of his drawers getting out the special magic lube that he kept. He tossed it behind you making you watch as the creature caught it.

  
  


“He’s sentient?” You asked, watching the creature apply it to it’s glowing shaft. Sure enough you could see the bulbous portion of the blaster’s cock. It had two ridges just under the tip of its shaft that you could see wrinkling while it stroked the cinnamon flavored lube over its red glowing thick length. Yeah, you were probably going to be worn out yourself when he was done with you. 

  
  


“well all the blasters are actually. this is sparky, then i got buddy an’ max. sparky won the pool when i came up with this little plan.” You were learning so much that he actually could do. And a lot more than he expected. 

  
  


“color?” Sans asked again, making you look up at him. 

  
  


“Green but only if you agree to let me help suck you off.” You responded to him making him chuckle.

  
  


“alright pet just be gentle. ‘m not sure how intense this is gonna be. gonna untie yur hands, it it gets too much just raise yur left hand and do the hand signal for stop.” Sans explained reaching forward so that he could undo the bonds that held your hands in place. Allowing you to stretch them in front of you so they were resting on the tops of his femurs. “remember signal stop with yur hand if this gets to be too much.”

  
  


You nodded your head before going to grip the top of his shorts. Pulling them down so that his hard magic was released from its confines. His ecto was quite detailed that you wanted to have him pose one time so that you could sketch it out. It wasn’t as long but it made up for it in width. The crimson glow it carried had deep undertones with each one of his ridges. With a little gold rounded stud that was just under the head. You licked your lips knowing that you enjoyed giving him head the most because he tasted like cherries that had been toasted over a bonfire. 

  
  


Moving forward you tilted his shaft towards your mouth placing a light kiss at the head of his shaft. “c’mon pet don’ tease me. i gave ya some good head. least ya can do is not tease me.” You heard Sans huff. He never was one for being teased. And you can attest to the last time he allowed you to dominate him in bed. 

“gentle boy. ‘member she is still small compared to ya.” You heard Sans murmur between purrs and groans of pleasure.

  
  


Pushing more of him into your mouth nearly choking as you felt something hot prod the entrance of your slit. That was definitely bigger than what you were used to. It was thicker than Sans even as it slid partially inside of you. You whimpered onto Sans feeling it pull out slightly. You tried to keep your attention focused on him so you could minstrate your tongue over him. Feeling as the hound pushed further into you. 

  
  


Your eyes looked up to see Sans had his attention focused behind you. Monitoring what the hound was doing. He looked down feeling the hound completely hilt itself inside of you. Sans placed both of his hands on your face. Lifting your head up off his shaft you let out a series of heavy pants. “color?” 

  
  


“Green.” You whispered, closing your eyes feeling so stuffed from the hound being buried inside of you. When you looked up you could see him smiling, his attention looking back at the hound behind you.

  
  


“yur green pal. give ‘er a good ride.” Was all he said before pushing your head down. Causing you to take the remainder of his length in your mouth. Using your hair as a handle so that he could move your mouth up and down his length. This was out of the norm, to let one of his blasters take advantage of you. You weren’t complaining one bit but you remember him stating that it was his magic taking advantage of you.

  
  


The hound moved out of you going all the way to the tip. It thrust forward so that the bones that it’s cock was attached to. You sucked in a deep breath through your nose as Sans pushed down on your head. “doin’ so good, pet. best get to work on my length. ya asked for me to do this for ya. least ya can do is keep up yur end.” Sans snickered. “an’ don’ worry i got this all bein’ filmed so ya can watch later.” He tilted his head to the side of the room. 

  
  


Looking over you could see that he had a camera set up on his computer desk. The three different screens that he had connected to his computer showing off the position you were currently in. The hound was thrusting into your body slowly while your face was pressed between Sans’ spread legs. You moaned at the visual admiring that Sans took the opportunity to record it for your pleasure. Pressing your hands on his femurs you started to work his cock with your mouth.

  
  


Sans really enjoyed when you dragged the flat base of your tongue over his piercing. It got Sans to let out a gasp of pleasure. His phalanges stroking against your scalp. You thrust your hips back into the hound’s thrusts. The hound growled in approval of your motions. You felt its tongue lightly lapping between your shoulder blades. 

  
  


The push and pulling motion of Sans and the hound did to your body had you feeling amazing. The hound had started to drool over your back letting it trail downwards. Coating you over the other orifice of your body. Sans snapped his finger getting the hound to extract itself from you. “since you’ve been such a good pet i think i should do somethin’ a bit nicer for ya. roll over sparky.” Sans called out moving so that your mouth wasn’t lodged around him.

  
  


He turned you so that you could see the hound laying on it’s back with that magical cock pointing upwards. Sans picked you up and carried you towards the hound tossing you over the top of it. Your hands gripped onto it’s rib cage while its front paws crossed over the small of your back locking you in place. It’s cock jutting forward so that it sealed itself inside of your folds again. Getting you to whimper from the sudden entrance. 

  
  


Bony hands gripped the back of your ass moving you in a rocking motion. Making sure that you felt every inch as it sunk into you. It felt amazing. You hadn’t expected to feel something else prodding against your ass. Looking over your shoulder you could see Sans grinning down at you. Winking in your direction but he didn’t move further. “Green.” You spoke giving him the okay. 

  
  


Sans nodded his head before pushing forward into your ass. It made you lower your head down on the hounds chest. Sans and the hound fucked you in time. Both of them making low growling noises that mixed with your whimpers. The three of you came at the same time but you had not been ready for how quickly the hound dissipated when it was finished. 

  
  


Sans gripped you pulling you back with him as he fell onto the floor over one of the piles of socks. “give me a bit, pet.” He spoke between heavy breaths. “just need to catch my breath. then we can go take a nice hot bath.” He chuckled moving to nuzzle his nasal cavity against your neck. “are ya okay?”

  
  


“I think I would love to try that again sometime.” You chuckled turning to rest the front of your body into his clothed ribs. “Thank you.”

  
  


Sans chuckled his hand moving under his bed. “good now can we talk about ya bein’ into tentacle porn? cause i think i can swin’ that.” Sans snickered making you look up to find he had taken your entire stash. Well shit… looks like you were going to have a busy few days off. 

  
  
  
  
  



End file.
